An E8400/E8500 because I have a wonderful overclocking motherboard laying 2 ft from me begging to be used. It's a Gigabyte EP45-UD3P which also offers me Crossfire capabilities which I need.
Agreed that 775 is a deteriorating socket there's no reason why a E8400 won't serve my needs until 6 or 7 months down the road, when I can come up with the cash to invest in Intel's latest.
Going with any thing else would constitute me having to purchase a new motherboard and would put me over my total $850 budget. (As of now I'm perfect in that budget, $500 for cpu, case, and GFX; $350 for the water cooling).
My goal here is overclocking potential. I know I can push 5GHz with this set up, I've seen it time and time again on xtremesystems.
I'm still managing to get by with a X2 5600 so I'll be pretty content with a E8400.